After more than a year of being mystified by the fact that when users who had logged-in to CB [1.9] tried to checkout orders in Virtuemart [1.1] shop, they had to log-in again [same in the back end] I finally realised what the issue was. My site has no www. in the domain name, but the Virtuemart configuration file defaults to including one. Once I realised that I could reset this in the VM Configuration>Security settings the problem disappeared. I can't believe it's taken so long to see what the problem was...duh
